1. Field of the Invention
The present invention relates to a separator used in a lithium ion secondary battery.
2. Description of the Related Art
Lithium ion secondary batteries which produce an electromotive force, by lithium doping/dedoping, have come into increasing use with the advance of portable electronic devices in recent years, because of their characteristically high energy density. In order to achieve even higher performance for lithium ion secondary batteries, it has been proposed to use separators comprising organic polymers which swell in, and retain, electrolyte solutions, such as polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA) or polyvinylidene fluoride (PVdF). Among these, PVdF or PVdF copolymers which are composed mainly of PVdF are considered to be most suitable from the standpoint of oxidation/reduction resistance, and some of these have been developed for practical use.
It is known that separators using such organic polymers have inadequate mechanical properties and are difficult to form into thin films, as compared with polyolefin fine porous films most commonly employed as separators. Polyolefin fine porous film reinforcement and nonwoven fabric reinforcement have been proposed as methods of reinforcement, and polyolefin porous film reinforcement is primarily used at present. On the other hand, the present inventors have found, and have disclosed in WO01/67536, that suitable control of the morphology of a separator with nonwoven fabric reinforcement can exhibit an overcharge protecting function and thereby markedly improve the safety during overcharge.
As nonwoven fabrics for reinforcement of such separators there have been proposed polyolefin-based materials (polyethylene, polypropylene) or heat-resistant materials such as aromatic polyamides, polyesters and the like. However, the mechanical properties of olefin-based nonwoven fabrics has presented a problem from the standpoint of thin film formation. It has therefore generally been considered more practical to use nonwoven fabrics composed of heat-resistant fibers of aromatic polyamides and polyesters.
Nonwoven fabrics employed for such purposes are exposed to organic solvent environments during the separator film-forming step and when incorporated into the battery and, consequently, the hitherto proposed nonwoven fabrics composed of heat-resistant fibers of aromatic polyamides and polyesters have been associated with problems because of their reduced mechanical properties in organic solvent environments, and hence unsatisfactory productivity.
As mentioned above, nonwoven fabric reinforcement can markedly improve safety during overcharge. However, an extra element is sometimes necessary to ensure safety against abnormal heat release by the battery due to various causes (nailing, external shorts, etc.). With overcharging as well, the battery can also conceivably experience abnormal heat release by Joule heat release when charging with a high current.